1. Field of the Invention
The present invention relates to filtration material, more particularly, filtration material that is suitable for air filtration in heating, ventilating, and air conditioning systems.
2. Discussion of the Art
The primary objective of a high performance air filter is to provide maximum particle removal while maintaining an acceptable pressure drop. Conventional fibrous filters obtain improved particle removal efficiency by using filaments having small diameter, which can be packed closely together. This close packing results in smaller voids between the fibers, which increases the probability that particles will be intercepted by a fiber. This increase in particle removal is, however, accompanied by an increase in pressure drop. Excessive pressure drops can restrict air flow and shorten filter life.
The use of electrostatic fibers can improve particle removal without affecting pressure drop or filter life. The charge on the fiber enhances particle removal efficiency by attracting particles or aerosols to the fiber. Therefore, a filter made from electrostatic fibers has a higher particle removal efficiency with the same pressure drop and dust holding capacity as a filter made from an uncharged web having the same mechanical construction.
Japanese patent application JP57-105217 A discloses air filter material comprising electret fibers of rectangular cross-section combined with staple, uncharged fibers of small diameter. The staple, uncharged fibers can be made of glass, polyester, polypropylene, or other fiber-forming materials. Pleated filters made from this media are not suitable for use with conventional heating, ventilating, and air conditioning systems because the pressure drop across the media is too great at air velocities normally encountered in such systems. Furthermore, additional processes are required, e.g., needle punching or addition of support layers, in order to realize strength characteristics approaching those suitable for filter materials for residential heating, ventilating, and air conditioning systems.
U.S. Pat. Nos. 3,016,599 and 4,118,531 describe articles formed from combinations of staple and meltblown fibers; however, none of these patents disclose filter materials suitable for residential heating, ventilating, and air conditioning systems.
U.S. Pat. No. 4,729,371 discloses that webs may be electrically charged to enhance their filtration capabilities, as by introducing charges into the fibers as they are formed or by charging the web after formation thereof; however, it is difficult to obtain a high level of charge by the process of that patent.